What are coffee grading screens?

Coffee grading screens are the primary tool for physical bean sizing in the coffee industry. These screens are essentially metal sieves with holes calibrated in 1/64th-inch increments. For example, a 'Screen 18' has holes that are 18/64 of an inch (approximately 7.1mm) in diameter. During the post-harvest milling process, green coffee is passed through a stack of these screens, allowing smaller beans to fall through while larger, more uniform beans are retained on the upper layers.

This process is vital because bean size directly influences how coffee roasts. If a batch contains a mix of very small and very large beans, the smaller ones will heat up and roast much faster than the larger ones, often leading to uneven roast profiles—some beans may be underdeveloped while others are scorched. By sorting beans into specific screen sizes, roasters can achieve a more uniform heat transfer, resulting in a cleaner, more predictable cup of coffee.

Different regions utilize these screens in various ways. In countries like Kenya, grading is highly standardized by size, where 'AA' beans are typically those retained on screens 17 and 18. In Colombia, the 'Supremo' designation refers to beans that are screen 17 and above. However, it is important to note that while size is a useful indicator of quality and consistency, it is not the only metric. High-quality coffee is also evaluated through defect counts and sensory cupping.

In practice, screen sizing helps importers and roasters maintain quality control. While a larger bean size is often associated with higher quality, it is not a universal rule; some smaller varieties, such as certain Ethiopian heirlooms, are prized for their flavor despite having smaller screen sizes. Ultimately, grading screens provide a standardized language for the trade, ensuring that the coffee you buy has been sorted for uniformity and potential performance in the roaster.